The effects of cold temperature on Xylella fastidiosa -infected Vitis vinifera grapevines

Xylella fastidiosa is a xylem-limited, gram-negative bacterium that causes Pierce's Disease (PD) in grapevines. Cold temperatures have been associated with the geographic limitations of PD. PD-infected vines can also be cured of infection after exposure to cold temperatures. Although the "cold curing" phenomenon has been documented, little is known about the physiological factors that mediate cold therapy.;To better understand the cold therapy phenomenon in planta, we planted control and Xylella fastidiosa infected Vitis vinifera "Pinot Noir" (PN) and "Cabernet Sauvignon" (CS) grapevines across four locations in Northern California over two years. We also simulated similar conditions for control and infected vines across four temperature conditions in cold rooms over two years. Disease curing rates and xylem sap composition was determined. Temperature data was used along with curing rate data to develop a model that can predict cold curing. These predictions could be used by growers to determine if their vineyards are at risk for sustaining overwintering infections, or if their vineyard will likely be cold cured of X. fastidiosa infection.;To better understand the effects plant changes on the survival of X. fastidiosa, in vitro assays examining changes in temperature, pH and osmolarity were conducted. Cells were grown in various buffers, media, and Vitis vinifera xylem sap. X. fastidiosa was exposed to temperatures ranging from -20°C to 28°C. At -5°C, the bacterium can survive in most buffers and media for two days and in some solutions can survive up to six weeks. At -10°C and -20°C, populations decreased quickly over the first 24 hours. In hourly sampling at 28°C over a 24 hour period, X. fastidiosa survived best in xylem sap collected from grapevines grown under field conditions.;We also investigated a possible treatment for Pierce's disease. Foliar and drench applications of the plant growth regulator abscisic acid (ABA) were applied to one-year-old potted Vitis vinifera 'Pinot Noir' and 'Cabernet Sauvignon' vines infected with Xylella fastidiosa, the bacterial pathogen that causes Pierce's disease (PD). A naturally occurring ABA and a synthetic ABA were applied, and both materials showed some effectiveness. Pinot Noir grapevines treated with the drench ABA treatments had significantly greater disease curing effects than the unsprayed, control plants. It has been demonstrated that plant phenolics have antimicrobial properties and we found a positive correlation between effective ABA treatment and total phenols.